Vacuum flask



FIG. 1 is a front elevational view of a vacuum flask showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a right side elevational view thereof;

FIG. 4 is a top, left side perspective view thereof; and

FIG. 5 is a top plan view thereof. 

The ornamental design for vacuum flask, as shown. 